In the glass processing industry, chemical tempering technology is often used to improve the strength of ordinary glass. The principle includes: placing glass or a glass product needed to be tempered in a molten salt bath containing potassium nitrate melted at a high temperature, soaking it for a period of time, and forming a compressive stress layer on the glass surface by replacing sodium and lithium ions of small ion radius in the glass with potassium ions of large ion radius in the molten salt bath containing potassium nitrate, so as to achieve the purpose of improving the glass strength.
Surface compressive stress (CS) is often used as an indicator by those skilled in the art to evaluate the chemical tempering effect of glass. Test results show that the higher the purity of potassium nitrate in the molten salt bath containing potassium nitrate is, the higher the CS value of the tempered glass will be, and the better the chemical tempering effect of the glass will be. On the contrary, when the purity of potassium nitrate in the molten salt bath containing potassium nitrate is lowered, the CS value of the tempered glass is reduced, and the chemical tempering effect of the glass becomes poor. The analysis results show that the main reason for the decrease of the CS value is that the molten salt bath containing potassium nitrate is diluted and contaminated by sodium and ions released from the glass due to ion exchange. With increasing area of glass treated with the molten salt bath containing potassium nitrate, the concentration of sodium and lithium ions in the molten salt bath containing potassium nitrate increases gradually, and the concentration of potassium nitrate in the molten salt bath containing potassium nitrate decreases gradually, resulting in the decrease of the CS value of the tempered glass.
At present, in order to obtain tempered glass with desired CS value, a commonly employed approach is to remove the purity-lowered molten salt bath containing potassium nitrate and replace it with fresh molten salt bath containing potassium nitrate. The drawbacks of this approach include the following.
1) The low-purity molten salt bath containing potassium nitrate is discarded as a waste. When cooled from about 400° C. to normal temperature, the molten salt bath containing potassium nitrate discarded as a waste changes from the molten state to a strongly hard mass that is difficult to be broken. If potassium nitrate discarded as a waste is intended to be recovered, it needs to be broken and melted at a high temperature by the potassium nitrate manufacturers before recovery and purification, which is accompanied by high energy consumption, waste of resources, and high cost.
2) The replacement of the molten salt bath containing potassium nitrate involves: shutting down the operation to clean the spent molten salt bath and the inner wall of the chemical tempering furnace, adding the salt bath containing potassium nitrate, melting by heating, and maintaining the temperature. The production has to be shut down for 4 to 5 days during the whole process from replacing the molten salt bath to reproduction, resulting in the decrease in the operation rate of the equipment (the ratio of possible yield of a machine to actual yield) and the decrease in the actual productivity.
3) The consumption of the molten salt bath containing potassium nitrate is a main cost in the production of chemically tempered glass. As the glass strength requirement is increased, the frequency of replacement of the molten salt bath containing potassium nitrate is increased, and the cost of the chemically tempered glass is accordingly increased.
Therefore, to reduce the frequency of replacement and improve the utilization of the molten salt bath containing potassium nitrate and thus reduce the cost and create economic benefits for an enterprise, there is a need to develop an on-line method for stabilizing surface compressive stress of chemically tempered glass.